format-parser: Fix parse_format_specifier() failure case.

format-parser: Fix parse_format_specifier() failure case.

When the current token is an identifier that takes the abstract syntax
of a format specifier, but its "name" part is not a valid format name,
parse_format_specifier() returned false but still consumed the token.
This is a potential issue for callers that want to report an error that
mentions the identifier, e.g. parse_primary(), although the current lexer
tends to retain the same identifier when the new token is not an
identifier.